1. Field of the Invention
This invention relates generally to a system for producing a multi-toned image in an electrostatographic printing apparatus. More particularly, this invention relates to an apparatus for enabling a photoconductive intermediate transfer member to produce toned images and receive toned images from additional imaging devices.
2. Description of Related Art
Generally, electrostatographic copying is performed by exposing an image of an original document onto a substantially uniformly charged photoreceptive member. The photoreceptive member has a photoconductive layer. Exposing the charged photoreceptive member with the image discharges areas of the photoconductive layer corresponding to non-image areas of the original document, while maintaining the charge in the image areas. Thus, a latent electrostatic image of the original document is created on the photoconductive layer of the photoreceptive member. Charged developing material is subsequently deposited on the photoreceptive member. The developing material may be a liquid material or a powder material. The developing material is attracted to the charged image areas on the photoconductive layer. This attraction converts the latent electrostatic image into a visible toned image. The visible toned image is then transferred from the photoreceptive member, either directly or after an intermediate transfer step, to a copy sheet or other support substrate to create an image which is permanently affixed to the copy sheet, providing a reproduction of the original document. In a final step, the photoconductive surface of the photoreceptive member is cleaned to remove any residual developing material to prepare the photoreceptive member for successive imaging cycles.
This electrostatographic copying process is well known and is commonly used for light lens copying of an original document. Analogous processes also exist in other statographic printing applications, such as, for example, ionographic printing and reproduction, where a charge is deposited on a charge-retentive surface in response to electronically generated or stored images.
In multi-color electrostatographic printing, rather than forming a single latent image on the photoconductive surface, successive latent images, corresponding to different colors, must be created. Each single color latent electrostatic image is developed with a corresponding colored toner. This process is repeated for a plurality of cycles. Each single-color toner image is superimposed over the previously transferred single-color toner image(s) when it is transferred to the copy sheet. This creates a multilayered toner image on the copy sheet. Thereafter, the multilayered toner image is permanently fixed to a copy sheet, creating a full-color copy.
In tandem color printing, four imaging systems are typically used. Photoconductive drum imaging systems are typically employed in tandem color printing due to the compactness of the drums. Although drums are used in the preferred embodiments, a tandem system alternately uses four photoconductive imaging belts instead of the drums. Each imaging drum system charges the photoconductive drum, forms a latent image on the drum, develops a toned image on the drum and transfers the toned image to an intermediate belt. In this way, yellow, magenta, cyan, and black single-color toner images are separately transferred to the intermediate transfer belt. When superimposed, these four toned images are capable of producing a wide variety of colors.
However, a user may desire a specialty color which cannot be properly produced using the conventional tandem imaging device. In such circumstances, the user must physically remove one of the imaging drum systems, replacing it with an imaging drum system set up to form a toned image in the specialty color. Such a process requires considerable interaction by the user. Alternatively, a fifth imaging drum system can be added for the specialty color. However, this increases both the machine size and the system cost.
Furthermore, because of size limitations of copying machines, the physical space within a copy machine can usually accommodate no more than four photoconductive drums. If the user desires a specialty color, the user must either obtain a copier specially designed to accommodate more than four photoconductive imaging drum systems or remove one of the photoconductive imaging drum systems each time the user wants to print using a specialty color. Such systems waste a great deal of time and effort. Furthermore, even if a fifth specialty color is not required, it is still desirable to reduce the size and cost in a multicolor intermediate transfer system.
A plurality of bias rollers or other similar conductive contacting members, with different potentials on the contacting members, may be used near the transfer zone of each photoconductive drum to tailor the electrostatic fields used in transferring. General "field tailoring" principals are known, as indicated in U.S. Pat. No. 5,198,864, the disclosure of which is incorporated herein by reference. The same general principals apply in transfers to a photoconductive intermediate belt when the belt substrate is semiconductive. Field tailoring helps reduce the unwanted electrostatic fields that otherwise extend across air gaps prior to intimate contact between the intermediate belt surface and the toner image on the surface of the drum.
Additionally, when using an intermediate belt transfer system, a toned image typically undergoes two electrostatic transfers before being fixed to a copy sheet. The first transfer occurs when each toned image is transferred from the photoconductive imaging drum system to the intermediate belt. The second transfer occurs when the intermediate belt transfers a multi-toned image to a copy sheet or another similar belt for transfer to the copy sheet. Because of the adhesive forces involved in transferring toned images, each transfer step can result in toner being lost or smeared. Typically, certain toner colors are found to be more easily lost or smeared in transfer systems than other color toners used in the transfer process. This is due to many factors related to toner design, such as different charge distributions, sizes, or material additives for charge or development control, causing different adhesion constants for the different toners. Different toner systems will have different high adhesion toner colors depending on the details of the toner materials of each color in the system. The number of transfers a toned image undergoes should therefore be minimized, especially for the least adhesive color toner in the system.